1. Field of the Invention
The present invention relates to resistance devices and more specifically to single component resistances of the unitary type which are simple to manufacture and of extreme accuracy and high tolerance.
2. Description of the Prior Art
In the design of current limiting circuits wherein the current to be limited is sensed by means of the voltage dropped across a known resistance, several problems are encountered by the circuit designers. One such problem is the limited availability of suitable resistance elements to accomplish optimum circuit function at a reasonable cost and in a desired or required small size.
Typically, conventionally available tubular shaped axial leaded wire wound resistors are employed in circuit applications in which the combination of nominal resistance value, resistance tolerance, power dissipation and mounting space are adequate for the purpose.
However, for those applications wherein only a very small or slight variation in resistance value from unit to unit is allowable and wherein nominal resistance value is very small, two terminal resistor elements become impractical due to the fact that the resistance of the device lead wires is a significant portion of the total resistance presented to the circuit by the resistance part.
This problem is usually solved by the employment of so called "KELVIN" type resistors. These resistive devices enable a much more precise current sense function by providing a second pair of leads for circuit interconnection. The secondary leads are connected to the resistive element such that the electrical resistance between these two additional lead connecting points is precisely controllable.
The in-circuit operation of the "KELVIN" resistor is obtained by passing the current to be measured through the entire resistive element which has a relatively imprecise resistance value due to the resistance of the current carrying leads. However, by measuring the voltage drop only across the precise value portion of the resistance, i.e. between the two additional "sense" leads, the resistance of these leads becomes insignificant. This is due to the fact that the "sense" leads do not pass the main input current.
This method of measurement works quite well. However, the components themselves are relatively costly and physically quite large for the function performed. Additionally, if the current sense function is to be applied to a pulsed or AC current of high frequency or fast rise time and/or fall time, then the component becomes even more costly since the inductance of conventional wire-wound resistors becomes prohibitive and the required resistances must be manufactured using the "KELVIN-VARLEY" process.
The "KELVIN VARLEY" technique involves two parallel windings of opposite winding direction in order to cancel most of the inductance which of course increases the cost of the device.